The present invention relates to a method and a device for open-loop control of single-phase or multiphase A.C. power controllers through phase-angle control of semiconductor valves.
Alternating current (A.C.) power controllers are used to provide for closed-loop control of the power supplied to an electrical load in an A.C. system. These power controllers are triggered through the phase-angle control of semiconductor valves. Particularly during the operation of an A.C. motor, changing operational conditions, such as different loading of the motor during start-up or lag phases when it is turned ON and OFF, require a closed-loop control of the power supplied to the motor to protect the power system, the motor and the driving gear from unnecessary loads.
British Patent 2 084 359 discloses a device for open-loop control of an A.C. power controller for an A.C. motor. This device is supposed to improve an unfavorable power factor caused, for example, by underloading of the motor. For this purpose, controllable semiconductor valves, in particular a triac, a bidirectional triode thyristor, or an antiparallel thyristor circuit, are assigned to the motor for each lag phase. These semiconductor valves enable power to be supplied in dependence upon the prevailing operating conditions of the motor by means of phase-angle control. The power factor is improved with this known device because the phase difference between the current and voltage is detected for each phase in a forward controlling element and reduced by properly increasing the ignition angle, i.e., the period of time that elapses between the current zero crossing and the point of ignition.
In the case of the known device, the instant of time of current zero crossing, which is determined by measuring the voltage applied across the triac, is used as the time reference for determining the point of ignition. This voltage is fed to a comparator, whose output states correspond to the circuit states of this triac. The current zero crossing corresponds then to an edge of the output signal from the comparator. From this edge and with the help of a monoflop, a strobe pulse is generated for a ramp voltage that is synchronized with the zero crossing of the supply voltage. The sampled value of the ramp voltage is subtracted from a reference voltage that is input externally by way of a potentiometer and transmitted to the inverting input of a differential amplifier, whose output voltage is fed, together with the ramp voltage to a further comparator. That second generator generates a primary firing signal via a downstream trigger pulse generator, when the ramp voltage exceeds the output voltage of the differential amplifier. Therefore, with this circuit arrangement, the phase shift between the motor current and the motor voltage, and thus the power factor, is stabilized at a value which is specified by the reference voltage set on the potentiometer.
Practice has shown, however, that there are problems when the instants of current zero crossings are determined by measuring the voltage drop across the semiconductor valve, reactions can occur as the result of induced voltages, particularly when there are inductive loads. These reactions make it more difficult to reliably determine the instants when the current goes to zero. Then, as a result of inductive reactions emanating from a continuously turning rotor, for example, it can happen that the voltage measured across the semiconductor valve does not reach the threshold value required to switch over the comparator, so that disturbances occur in the course of the firing sequence. In multiphase A.C. motors, direct-current components can then build up and result in the motor being subjected to shock or sudden impact loads.
To substantially eliminate these types of disturbances, one must select the lowest possible threshold value for the comparator. However, this type of solution entails increasing sensitivity and in turn susceptibility to faults caused by system disturbances and inductive voltage surges in the control of inductive loads.
Therefore, in view of the sensitivity of the measuring device, a compromise must always be made when determining a reference instant for the time control of the phase angles.